This is a method for making turbine blades for combustion turbines, including aircraft turbines, marine turbines, and land-based gas turbines. This invention utilizes a two step solidification to produce a fine grained (non-directionally solidified) structure in the root section and a directionally solidified structure in the airfoil section.
Gas turbine engines operate by extracting energy from high temperature, high pressure gas as it expands through the turbine section. The actual rotating components which are driven by the gas are manufactured from nickel-based superalloys and are commonly known as blades. They consist, as shown in FIG. 1, of a contoured airfoil which is driven by the hot gas stream and of a machined root which connects to the turbine rotor. Due to the nature of the carnot cycle, gas turbines operate more efficiently at higher temperatures and there has thus become a demand for materials which are able to withstand higher temperatures. The major mechanical modes of failure for turbine blades, such as aircraft engines and in land-based turbine generators, at high temperatures have been thermal fatigue and the lack of creep rupture resistance. Both of these problems may be reduced by elimination of grain boundaries which are transverse to the major stress axis. Thus, single crystal and directionally solidified blades are known to display significantly improved high temperature strength.
While large grain sizes improve the desired properties in the very high temperature regime, at low temperatures certain mechanical properties are improved by lower grain size. Specifically, the root section of a turbine blade runs at considerably lower temperature than the airfoil and is, essentially, subjected to fatigue loading. Consequently, the optimum structure for airfoil and root sections of the blades are very different and, in conventional airfoils, some compromise must be accepted in one of these sections. The optimum properties would be obtained if a hybrid blade structure were produced with a directionally solidified airfoil and a fine grained root section.
In U.S. Pat. No. 4,184,900, issued Jan. 22, 1980 to Erickson et al., two different directionally solidified sections are produced to obtain different properties in the airfoil and root sections. In U.S. Pat. No. 3,790,303, issued Feb. 5, 1974 to Endres, a eutectic alloy is used to produce a hybrid turbine blade (bucket) having an airfoil which is directionally solidified and a non-oriented structure in the root, the eutectic composition avoiding composition inhomogenuities which would result if non-eutectic compositions were used in such a method.